1. Field of the Invention
The present invention is related to optical communications systems and is particularly directed to a method and apparatus for dynamic polarization control that is useful in a system for compensating for polarization mode dispersion in optical fibers.
2. Background
Optical communications systems employing optical networks are critical to meeting the ever-increasing demand for high speed communications systems capable of attaining higher data transfer rates. Such systems generally employ wavelength division multiplexing (“WDM”) whereby a plurality of optical signals are simultaneously transmitted as separate channels in a single optical fiber. As higher data rates are implemented, polarization mode dispersion (“PMD”) becomes a critical issue, such that PMD is currently regarded as one of the dominant obstacles to increased data transfer rates. It is generally understood that PMD begins to become a factor at data transfer rates of 10 Gb/s and compensation for PMD is critical to avoid data loss at rates exceeding 40 Gb/s.
Unlike chromatic dispersion, PMD is not fixed and changes constantly as a function of many time-variable factors, drifting in time scales that vary from a few milliseconds to several hours. PMD arises due to birefringence in an optical fiber that causes light of different polarization to travel at different speeds. Although, theoretically, a perfectly round optical fiber is not birefringent, such birefringence may be introduced when the fiber is subject to uneven mechanical stress, for example, due to bending, compression, heating, etc. As a result, static compensation of PMD is not practical. Thus, an effective system for compensating for PMD should be dynamic and capable of responding to changes in the polarization properties of the transmitted signals in a matter of a few milliseconds. Moreover, PMD can vary from channel to channel, and so a system for compensating for PMD should be controllable on a channel-by-channel basis. Preferably, apparatus used for PMD compensation should be compatible and readily integrated with other devices in a WDM optical network, such as multiplexers (MUX), demultiplexers (DEMUX), variable optical attenuators (VOA), planar optical amplifiers, optical switches, etc. Finally, PMD compensation apparatus should be relatively easy and inexpensive to construct. A dynamic polarization controller is a key component of a system to compensate for PMD.
While considerable effort has gone into studying PMD and its causes, it is believed that less effort has been directed to solving the problem, particularly in a way that could be usefully implemented in WDM optical networks on a cost-effective basis. Thus, there is a need for improved apparatus and methods for dynamically compensating for PMD in optical networks, and such is an object of the present invention. Moreover, there is a need for improved polarization controllers for use in PMD compensation system and in other applications.